A core of a nuclear reactor, such as a fast neutron nuclear reactor cooled by liquid metal, comprises detachable assemblies disposed vertically and maintained in position by a support or bolster in which the lower parts or feet of the assemblies of the core are engaged, inside sleeves having a vertical axis disposed in a network corresponding to the network of the assemblies in the core. The vertical sleeves, termed pillars, join the upper part to the lower part of the bolster which is in the form of a hollow structure into which the coolant fluid of the reactor is injected, this fluid usually being constituted by liquid sodium. Each of the pillars comprises openings for the passage of the liquid sodium, corresponding openings in the foot of the assembly being aligned with the openings of the pillar. The liquid sodium can consequently travel to the assemblies in the upward direction for cooling said assemblies.
The fuel assemblies comprise, above their foot of generally cylindrical shape which is engaged in the sleeve, a prismatic part usually having a hexagonal section and terminating in its upper part in a head permitting the seizure of the assembly for its handling and possibly ensuring the upper neutronic protection of the assembly.
The assemblies constituting the core of a fast nuetron reactor are of several different types and have a predetermined position in the core. Some of these assemblies are fuel assemblies in which the power of the core is created, others of the assemblies being fertile and capable of ensuring a certain regeneration of the nuclear fuel, and still other assemblies being absorbent of various types for regulating the power or effecting urgent stoppages of the reactor.
When effecting the first charging of the core of the reactor, it is necessary not only to place each of the assemblies in a defined position in the core, but also to correctly and precisely orient the assemblies relative to one another owing to their prismatic shape, the hexagonal sections of these assemblies being imbricated so as to constitute the cross-section of the core. It is also necessary to simultaneously place the opengings provided in the foot of the assembly into alignment with the openings provided in the pillars of the bolster to obtain satisfactory conditions for the circulation of the coolant fluid as it enters the core.
In prior art designs of cores of fast neutron nuclear reactors, the feet of the assemblies are free to rotate about the axis of the pillar, all the corresponding foot and pillar sections bearing against one another being circular. When they are placed in position in the core, the assemblies are suspended from the grab of a handling machine so as to be substantially free to rotate about their vertical axis. The sole limit to the rotation of the assembly is provided by the friction of the bearing members for the suspension of the assembly from the grab under the effect of the weight of the assembly.
In order to ensure correct orientation of the assemblies relative to one another, there are provided around the assembly, in the part of the foot connected with the hexagonal body, contact surfaces having the shape of cams or shoes adapted to cooperate with contact surfaces of corresponding shape on the neighboring assemblies.
The constitution of the network of assemblies corresponding to the first core introduced in a new reactor is by a series of operations for achieving a perfect relative orientation of the assemblies of the network relative to one another.
Before the filling of the vessel with liquid sodium, a complete network is constituted with false assemblies having the same geometry as the true assemblies. Each false assembly is placed in position manually, and then an in situ visual inspection permits checking that the sodium passage openings in the foot of the assembly are perfectly aligned in with the corresponding openings of the pillar. This positioning and this checking can easily be carried out, since the vessel of the reactor in which the core is constituted is at that time in a normal atmosphere of air.
When all the false assemblies have been placed in position, there is available a complete network which acts as a reference for all the other handling operations which may be effected on the assemblies, and in particular for the charging of the first core after the vessel has been filled with liquid sodium. The false assemblies are replaced by the true assemblies by successive substitutions, each assembly taking the place of a false assembly having exactly the same geometry, and in particular identical contact and orientation surfaces.
When an assembly is introduced in a cavity consituted by six adjacent assemblies, it is possible to correct a slight angular offset of this assembly by cooperation of the corresponding orientation surfaces. However, the risk of a progressive angular offset of the assemblies relative to one another is not completely eliminated. Such a progressive offset may result in difficulty in the introduction of an assembly or in a large disalignment between the openings of the pillars and the openings of the foot of the assembly. In this case, the passage of the coolant liquid sodium in the assembly may be considerably affected.
In the course of the charging by progressive substitution of the assemblies of the first charge for the false assemblies, the position of each of the assemblies in the core is located in a precise manner by conventional primary handling means of the reactor.
The charging operations of a fast neutron nuclear reactor are therefore long and difficult to carry out, and the adaptation of the assemblies requires the formation of orientation shoes whose profiles are very complex.
Furthermore, the self-orienting shoes do not always perform their function, in particular when the assemblies are deformed after a certain irradiation time in the reactor.
Difficulties are also related to the design of the seizing head of the assembly and its connection with the hexagonal case constituting the body of the assembly.
Even in the case where they do not ensure the upper neutronic protection of the assembly, these seizing heads are massive and the design is delicate, bearing in mind the considerable thermal disturbances prevailing in the upper part of the assemblies.